This invention relates to formulations and methods for enhancing the efficacy of exogenous chemicals used in treating plants. An exogenous chemical, as defined herein, is any chemical substance, whether naturally or synthetically derived, which (a) has biological activity or is capable of releasing in a plant an ion, moiety or derivative which has biological activity, and (b) is applied to a plant with the intent or result that the chemical substance or its biologically active ion, moiety or derivative enter living cells or tissues of the plant and elicit a stimulatory, inhibitory, regulatory, therapeutic, toxic or lethal response in the plant itself or in a pathogen, parasite or feeding organism present in or on the plant. Examples of exogenous chemical substances include, but are not limited to, chemical pesticides (such as herbicides, algicides, fungicides, bactericides, viricides, insecticides, aphicides, miticides, nematicides, molluscicides, and the like), plant growth regulators, fertilizers and nutrients, gametocides, defoliants, desiccants, mixtures thereof, and the like.
Exogenous chemicals, including foliar-applied herbicides, have at times been formulated with surfactants, so that when water is added, the resulting sprayable composition is more easily and effectively retained on the foliage (e.g., the leaves or other photosynthesizing organs) of plants. Surfactants can also bring other benefits, including improved contact of spray droplets with a waxy leaf surface and, in some cases, improved penetration of the accompanying exogenous chemical into the interior of leaves. Through these and perhaps other effects, surfactants have long been known to increase the biological effectiveness of herbicide compositions, or other compositions of exogenous chemicals, when added to or included in such compositions. Thus, for example, the herbicide glyphosate (N-phosphonomethylglycine) has been formulated with surfactants such as polyoxyalkylene-type surfactants including, among other surfactants, polyoxyalkylene alkylamines. Commercial formulations of glyphosate herbicide marketed under the trademark ROUNDUP.RTM. have been formulated with a surfactant composition based on such a polyoxyalkylene alkylamine, in particular a polyethoxylated tallowamine, this surfactant composition being identified as MON 0818. Surfactants have generally been combined with glyphosate or other exogenous chemicals either in a commercial concentrate (herein referred to as a "coformulation"), or in a diluted mixture that is prepared from separate compositions, one comprising an exogenous chemical (e.g. glyphosate) and another comprising surfactant, prior to use in the field (i.e., a tank mix).
Various combinations of exogenous chemicals and surfactants or other adjuvants have been tested in the past. In some instances, the addition of a particular surfactant has not produced uniformly positive or negative changes in the effect of the exogenous chemical on the plant (e.g., a surfactant that may enhance the activity of a particular herbicide on certain weeds may interfere with, or antagonize, the herbicidal efficacy on another weed species).
Some surfactants tend to degrade fairly rapidly in aqueous solutions. As a result, surfactants that exhibit this property can only be used effectively in tank mixes (i.e., mixed with the other ingredients in solution or dispersion in the tank soon before spraying is to occur), rather than being coformulated in an aqueous composition with the other ingredients in the first instance. This lack of stability, or inadequate shelf-life, has hindered the use of certain surfactants in some exogenous chemical formulations.
Other surfactants, though chemically stable, are physically incompatible with certain exogenous chemicals, particularly in concentrate coformulations. For example, most classes of nonionic surfactant, including polyoxyethylene alkylether surfactants, do not tolerate solutions of high ionic strength, as for example in a concentrated aqueous solution of a salt of glyphosate. Physical incompatibility can also lead to inadequate shelf-life. Other problems that can arise from such incompatibility include the formation of aggregates large enough to interfere with commercial handling and application, for example by blocking spray nozzles.
Another problem that has been observed in the past is the effect of environmental conditions on uptake of an exogenous chemical composition into foliage of a plant. For example, conditions such as temperature, relative humidity, presence or absence of sunlight, and health of the plant to be treated, can affect the uptake of a herbicide into the plant. As a result, spraying exactly the same herbicidal composition in two different situations can result in different herbicidal control of the sprayed plants.
One consequence of the above-described variability is that often a higher rate of herbicide per unit area is applied than might actually be required in that situation, in order to be certain that adequate control of undesired plants will be achieved. For similar reasons, other foliar-applied exogenous chemicals are also typically applied at significantly higher rates than needed to give the desired biological effect in the particular situation where they are used, to allow for the natural variability that exists in efficiency of foliar uptake. A need therefore exists for compositions of exogenous chemicals that, through more efficient uptake into plant foliage, allow reduced use rates.
Many exogenous chemicals are commercially packaged as a liquid concentrate that contains a significant amount of water. The packaged concentrate is shipped to distributors or retailers. Ultimately the packaged concentrate ends up in the hands of an end user, who further dilutes the concentrate by adding water in accordance with label instructions on the package. The dilute composition thus prepared is then sprayed on plants.
A significant portion of the cost of such packaged concentrates is the cost of transporting the concentrate from the manufacturing site to the location where the end user purchases it. Any liquid concentrate formulation that contained relatively less water and thus more exogenous chemical would reduce the cost per unit amount of exogenous chemical. However, one important limit on the ability of the manufacturer to increase the loading of the exogenous chemical in the concentrate is the stability of that formulation. With some combinations of ingredients, a limit will be reached at which any further reduction of water content in the concentrate will cause it to become unstable (e.g., to separate into discrete layers), which may make it commercially unacceptable.
Accordingly, a need exists for improved formulations of exogenous chemicals, particularly herbicides, that are stable, effective, less sensitive to environmental conditions, and permit the use of reduced amounts of exogenous chemical to achieve the desired biological effect in or on plants. A need also exists for stable liquid concentrate formulations of exogenous chemicals that contain less water and more exogenous chemical than prior art concentrates.